After the explosion of each mining charge, wedgers and levers are employed, to drag away and break down what has been shattered.
Wherever the rock is tolerably hard, the use of gunpowder is more economical and more rapid than any tool-work, and is therefore always preferred. A gallery, for example, a yard and a half high, and a yard wide, the piercing of which by the hammer formerly cost from five to ten pounds sterling the running yard, in Germany, is executed at the present day by gunpowder at from two to three pounds. When, however, a precious mass of ore is to be detached, wnen the rock is cavernous, which nearly nullifies the action of gunpowder, or when there is reason to apprehend that the shock caused by the explosion may produce an injurious fall of rubbish, hand-tools alone must be employed.
When the rocks which cover valuable minerals are not of very great hardness, as happens generally with the coal forma tion, with pyritous and eluminous slates, sal gem, and some other minerals of the secondary strata, the borer is employed with advantage to ascertain their nature. This mode of investigation is economical, and gives, in such cases, a tolerably exact insight into the riches of the interior.
The mode of working mines is two fold ; by open evocations, and subter ranean.
Workings in the open airpresent few difficulties, and occasion little expense, unless when pushed to a great depth. They are always preferred for working deposits little distant from the surface; where, in fact, other methods cannot be resorted to, if the substance to be raised be covered with incoherent matters. The only rules to be observed are, to arrange the workings in terraces, so as to facili tate the cutting down of the earth ; to transport the ores and the rubbish to their destination at the least possible ex pense, and to guard against the crumb ling down of the sides. With the latter view, they ought to have a suitable slope; or to be propped by timbers whenever they are not quite solid.
Open workings are employed for valu able clays, sands, as also for the alluvial soils of diamonds, gold, and oxide of tin, bog iron ores, &c., limestones, gypsums,
building stones, roofing slates, masses of rock-salt in some situations, and certain deposits of ores, particularly the specu lar iron of the island of Elba, the masses of stanniferous granite of Gayer, Alten berg, and Seyfen, in the Ertzgeberge, a chain of monntai ts between Saxony and Bohemia •, the thicit veins or masses of black oxide of iron of Nordmarch, Dan nemora, &e., in Sweden ; the mass of cupreous pyrites of Menu's, near Dron theim in Norway ; several mines of iron, copper, and gold in the Ural mountains, &c.
Subterranean workings may be con veniently divided into five classes, viz.:— 1. Veins or beds, much inclined to the horizon, having a thickness of at least two yards.
2. Beds of slight inclination, or nearly horizontal, the power or thickness of which does not exceed two yards.
8. Beds of great thickness, but slight ly inclined.
4. Veins, or beds highly inclined, of great thickness.
5. Masses of considerable magnitude in all their dimensions.
Subterranean mining requires two very distinct classes of workings; the prepara tom and those for extraction.
The preparatory consist in galleries, or In pits and galleries destined to conduct the miner to the point most proper for attacking the deposit of ore, for tracing it all round this point, for preparing chambers of excavation, and for concert ing measures with a view to the circula tion of air, the discharge of waters, and the transport of the extracted minerals. If the vein or bed in question be placed in a mountain, and if its direction forms a very obtuse angle with the line of the slope, the miner begins by opening in its side, at the lowest possible level, a gallery of elongation, which serves at once to give issue to the waters, to csxplore the deposit through a consider • _extent, and then to follow it in another . i ection; but to commence the real minim: opera tions, he pierces either shafts or alleries, according to the slope e deposit, across the first gallery.